Evolutionary radiations of cushion plants on the Qinghai-Tibet Plateau: Insights from molecular phylogenetic analysis of two subgenera of Arenaria and Thylacospermum (Caryophyllaceae)

Cushion plants exhibiting adaptive convergence in cold and dry environments are keystone and foundation species in the alpine/subnival habitat on the Qinghai-Tibet Plateau (QTP). To date, little attention has been paid to the molecular phylogeny, origin, and biogeography of cushion plants on the QTP. We investigated the molecular phylogeny of classic cushion Arenaria subg. Dolophragma, A. subg. Eremogoneastrum, and Thylacospermum on the QTP, within the framework of the Caryophyllaceae. A new Thylacospermum-Dolophragma clade was identified using combined plastid markers (rps16, matK, trnL-trnF, trnS-trnfM) and nuclear ribosomal DNA. Molecular divergence dating suggested that Thylacospermum and A. subg. Dolophragma originated in the middle to late Miocene (11.68 Ma). Combined with ancestral range reconstruction, there is an indication that all species of cushion Arenaria (A. subg. Dolophragma, A. subg. Eremogoneastrum) originated during/after the late Pliocene, and the QTP was their ancestral area. Furthermore, ecological niche modeling showed that the areas occupied by the three studied cushion plants during the last glacial maximum (LGM) was broader than that of their present distribution, implying a reduction in their range after the LGM. This evidence clearly illustrates that multistage uplift of the QTP and bordering mountains since the Miocene associated with climatic change (worldwide cooling, aridification in Central Asia, Quaternary glaciation) played a role in triggering and facilitating the speciation and/or evolutionary radiations of the species studied.